This dialog lets you specify which combinations of attribute levels (or attributes altogether) may not be shown together in pairs (and for ACA questionnaires in calibration concept questions).
Prohibitions, if at all possible, should be avoided. Specifying unnecessary or excessive prohibitions is one of the most common mistakes. The problem usually begins when either the analyst (or the analyst's client) notices that some product combinations displayed during the interview are not realistic, given what currently exists in the market. Sometimes a product is shown with all the best features at the lowest price; or two attribute levels that would not naturally occur in the real world are paired together. The inclination is simply to prohibit such combinations. We urge you to exercise restraint when considering prohibiting pairs.
Too many prohibitions can lead to imprecise part-worth estimation. It is better to prompt respondents that they will see combinations during the interview that are not yet available in the market or that seem unlikely. You can urge respondents to answer as if these products were actually available today.
How Many Prohibitions Can I Safely Add?
A common question that users ask is "how many prohibitions can I specify without seriously damaging the results of my study?" This cannot be answered without more information. For it is not the sheer number of prohibitions but the specific pattern of prohibitions that more directly affects the degree of correlation among the attribute levels in the design matrix, and thus the design efficiency.
Assume that the researcher wants to specify 3 prohibitions between a 2-level attribute and a 3-level attribute. There are just 6 possible combinations that can occur when combining those two attributes. If the researcher prohibits 3 of those combinations from occurring, this eliminates half of the possible combinations between those attributes. There would probably be less damage to the efficiency of the design if three total prohibitions were allocated across three separate attribute combinations (one per pair of attributes).
As a side note, the researcher in this instance may decide to specify the three non-prohibited combinations of those two attributes as a single attribute, rather than as two separate (and not independent) attributes. The main drawback of this approach is that after combining these two attributes, the researcher will not be able to compute the attribute importance or the relative part-worths of each attribute independently.